That is the first 16 bore I have seen that was marked 2 9/16" for the chamber length. How about you Smallbore as you see far more guns that I. The 1 ounce proof mark is for the standard 2 1/2" cartridge normally. I would refrain from using 2 3/4" ammo in it. With the barrel length and choke it is unlikely to be a pigeon gun as I first thought but it is certainly built on that style. Lovely gun. Lagopus.....